Caprolactam can be prepared by Beckmann rearrangement of cyclohexanone oxime. Such Beckmann rearrangement can be carried out by admixing cyclohexanone oxime to a reaction mixture comprising caprolactam, sulfuric acid and SO3. In such process the sulfuric acid and SO3 is a catalyst for the conversion of cyclohexanone oxime towards caprolactam. Such conversion is known to occur instantaneously.
Such a process is described in U.S. Pat. No. 3,953,438. In this process cyclohexanone oxime containing 4.9 wt. % water and oleum containing 25 weight percent SO3 are continuously fed to a first rearrangement zone containing a circulating reaction mass such that the circulating reaction mass having a sulfuric acid +SO3 to caprolactam weight ratio of 1.45 and a SO3 content of 5.9 weight percent. A portion of the reaction mass of the first catalytic rearrangement zone equivalent to the feed thereto is continuously removed and fed to a second rearrangement zone containing a circulating reaction mass. Cyclohexanone oxime containing 4.9 wt. % water is also continuously fed to the circulating reaction mass of the second rearrangement zone. The circulating reaction mass of the second rearrangement zone has a sulfuric acid +SO3 to caprolactam weight ratio of 1.14 and a SO3 content of 3.2 weight percent. A portion of the reaction mass of the second catalytic rearrangement zone equivalent to the feed thereto is withdrawn.
In U.S. Pat. No. 3,953,438 it is described that the oxime may contain up to 6 wt. % water. Although a value of 0.1 wt. % water is mentioned as a lower limit, in fact, the process disclosed in this patent is carried out at high water content, i.e. in all the examples oxime containing 4.9 wt. % of water is used. No method is given how to reduce the water content of the oxime.
In the process of U.S. Pat. No. 3,953,438 oleum having relatively high oleum strength is needed. Hence large quantities of SO3 are needed to obtain the desired yield to caprolactam.
The object of the invention is to improve the yield while using relatively small amounts of SO3.
This object is achieved in that the process further comprises obtaining the cyclohexanone oxime that is fed to the reaction mixtures by:    1) preparing an organic medium comprising cyclohexanone oxime dissolved in an organic solvent;    2) separating, by distillation, cyclohexanone oxime from said organic medium